There has been a significant research and development effort directed toward defining improved methods and materials for commercial manufacture of microelectronics devices. Generally the most technically intensive aspects of the manufacturer of such devices are associated with the active electrical components themselves. But the inherently delicate nature of such devices and their susceptibility to moisture and other elements of their operating environment requires that the active components be encased with a protective inert material. Such encasing or encapsulating material must not only protect the device from mechanical shock loads incurred in handling but it must also provide an electrically nonconductive environment for the device protecting it from light and moisture. Moreover, the encapsulant material must serve as a thermally conductive medium so that heat generated during operation of the encapsulated device is readily dissipated to minimize thermal stress.
Generally, many polymeric resins have been used commercially for microelectronic device encapsulation. Filled epoxy resins have been used most extensively. They have been found to provide the requisite physical/chemical characteristics, and durability required by the electronics industry. Yet in spite of the popularity epoxy resins have enjoyed commercially as a microelectronics device encapsulating resin, epoxy resins prepared for such applications are expensive, and they require extended cure times and post-cure processing. The use of epoxy resin encapsulants suffers as well from the costs of the extra labor/equipment required for transfer molding processes. Moreover, although much effort has been directed to the formulation of epoxy resin encapsulants having low coefficients of thermal expansion and low shrinkage during resin cure, stress-induced failure of fragile microelectronics devices during the encapsulation process remains as a significant problem in commercial device manufacturing operations using epoxy resin encapsulants.
Some of the disadvantages of the use of epoxy resins for encapsulation of microelectronics devices can be overcome by the use of filled polyester resins that can be delivered to molds using conventional injection molding technology. Polyester resins are generally less expensive, and they have a relatively rapid cure rate (and require no post-cure processing). Notwithstanding those well-recognized characteristics, polyester resins have not been accepted by the electronics industry as a suitable substitute for the more expensive thermosetting epoxy resins.
Polyester resins are known to have a high degree of shrinkage, and when such is not controlled, the stress imposed on an encapsulated device by a shrinking resin encapsulant can result in high device failure rates. It is known in the art that the shrinkage of polyester resins during cure (polymerization) can be controlled by, for example the addition of solid fillers and/or certain thermoplastic resins. Yet even with such shrink control technology available, polyester resins still have found little or no commercial acceptance for microelectronics encapsulation applications.
One disadvantage that state-of-the-art low viscosity thermoplastic-resin-modified polyester formulations still suffer when compared to their filled epoxy resin counterparts is their coefficient of thermal expansion (CTE), a physical property that bears significantly on the performance of device encapsulants under conditions of thermal cycling inherent in many microelectronics applications. A low CTE more aligned with the CTE of the device components themselves minimizes cracking of the encapsulant where it interfaces conductor leads or integrally molded metallic lead frames. State-of-the-art low viscosity (&lt;8.times.10.sup.6 centipoise) thermosetting polyester formulations containing shrink controlling amounts of thermoplastic resins cure to plastic matrices commonly exhibiting coefficients of thermal expansion of about 30.times.10.sup.-6 cm/cm/.degree.C. or higher, often more than 50.times.10.sup.-6 cm/cm/.degree.C. Some of the low-stress-formulated epoxy resin encapsulants on the other hand are reported to exhibit coefficients of thermal expansion of less than 30.times.10.sup.-6 cm/cm/.degree.C.
One other disadvantage noted in the art for use of polyester resins as encapsulants for microelectronics devices is that devices encapsulated with state-of-the-art polyester formulations generally exhibit more susceptibility to moisture related failure; an incomplete seal forms between the outer surface, e.g., of the leads, and the encapsulating resin. There has been some effort to address that problem--see Kaplan U.S. Pat. No. 4,327,369. But again, such efforts have failed to produce polyester resin formulations which meet the demanding standards of the microelectronics manufacturing industry. Thus, there is still a need for development of improved polyester resin formulations which can be used as economical substitutes for epoxy resins in commercial microelectronics device encapsulation applications.
Thus it is one object of this invention to provide an injection moldable polyester resin formulation which cures at low mold pressures to a plastic matrix having a low coefficient of thermal expansion.
It is another object of this invention to provide a phase stable, low viscosity, filled polyester resin formulation containing a low profile thermoplastic resin and thermally curable to a plastic matrix having good thermal conductivity and a coefficient of thermal expansion of less than 30.times.10.sup.-6 cm/cm/.degree.C.
Another object of this invention is to provide a filled polyester resin formulation substantially free of mold release agents which when used as an encapsulant for electronics devices exhibits commercially acceptable mold release characteristics and provides polyester resin encapsulated devices exhibiting enhanced resistance to moisture related failure.
It is still a further object of this invention to provide a filled, phase stable, thermoplastic-resin-modified polyester resin formulation that has a molding viscosity which allows use of the resin formulation in conventional pressure molding apparatus.
Still another object of this invention is to provide a low viscosity thermosetable polyester resin formulation containing a combination of polyethylene and a low shrink thermoplastic resin and thermally curable to a plastic matrix having coefficient of thermal expansion of a value less than 30.times.10.sup.-6 cm/cm/.degree.C.
In yet another object of this invention there is provided an improved process for preparing plastic encapsulated microelectronics devices with significant reduction in stress-induced device failures by using an economical, low viscosity, filled polyester resin formulation containing a low profile thermoplastic resin additive or a combination of polyethylene and a low shrink resin additive in an amount effective to provide a resin composition which can be thermoset to a plastic matrix having good thermal conductivity and a coefficient of thermal expansion of about 10.times.10.sup.-6 to about 30.times.10.sup.-6 cm/cm/.degree.C.
In still another embodiment of this invention there is provided a microelectronic device encapsulated in a polyester matrix substantially free of conventional mold release agents and having a coefficient of thermal expansion less than 30.times.10.sup.-6 cm/cm/.degree.C.
Those and other related objects which will be apparent to those of ordinary skill in the art are accomplished in accordance with this invention by preparation and use of a novel thermosetting polyester resin formulation comprising an unsaturated polyester, a low profile thermoplastic resin or a combination of polyethylene and a low shrink thermoplastic resin, an unsaturated monomer capable of cross linking the unsaturated polyester, an inert filler and a free radical initiating agent in an amount effective to polymerize the resin composition to provide a filled plastic matrix having a coefficient of thermal expansion less than 30.times.10.sup.-6 cm/cm/.degree.C. In preferred embodiments, the present resin composition is further characterized by low viscosity, less than 8.times.10.sup.-6 centipoise at normal ambient temperature, allowing it to be handled/molded in conventional pressure molding protocols. The low injection pressure allowed by the low viscosity of the present formulations not only reduces mold maintenance but also allows molding of devices having delicate, fine wires exposed, as is typical on many semi-conductor devices, without damage to the device.
The polyester resin formulations of this invention provide cost savings for electronic device encapsulation applications not only through their utilization of cost efficient polyester resins, but also by virtue of the reduced labor cost associated with pressure molding operations, and more significantly, the occurrence of fewer stress-induced device failures. The high thermal conductivity and the low coefficient thermal expansion of the cured encapsulant matrix achieved by molding the present low viscosity, filled, polyester resin formulations modified with low profile thermoplastic resin additives or a combination of polyethylene and a low shrink additive, find no antecedent in the art relating to injection moldable polyester formulations. While filled polyesters curable to plastic matrices having comparable coefficients of thermal expansion have been reported in the art, such are reported for the glass fiber-filled polyester molding formulations which have viscosities which would not allow use of those formulations for microelectronics device encapsulation applications using the low pressure molding protocol specified for use of the preferred low viscosity formulations of the present invention. The combination of properties exhibited by the improved polyester resin formulations of this invention makes them uniquely adapted for microelectronics device encapsulation. The present resin formulations have low molding viscosity (and concomitantly low molding pressures), and good phase stability. The cured resins exhibit low coefficient of thermal expansion, low modulus, favorable mold release characteristics without added mold release agents, and good moisture resistance of the encapsulated devices, all utilizing low cost (vs. epoxy resins) polyester resin components.